Oriented Polystyrene (OPS) is produced commercially by biaxial orientation processes. Biaxially orientation of glassy, rigid styrenic polymers makes the resultant sheet material flexible and tough. Although flexibility can be achieved by other techniques, such as incorporation of butadiene rubber or copolymers; these materials are expensive, difficult to process and usually reduce light transmission and clarity.
Conventional OPS processing is described by Bikales in "Extrusion and other plastics operations", Wiley-Interscience, 1971, pp. 103-132. Other prior art processes are disclosed by Beck et al in U.S. Pat. No. 4,980,101, which describes manufacture of semipermeable membranes employing a non-solvent mineral oil. During orientation the membrane is heated to temperature between the glass transition temperature and melting point of syndiotactic polystyrene. A type of foam sheet orientation is disclosed in U.S. Pat. No. 5,356,944, wherein Blythe et al suggest addition of mineral oil to polystyrene for lowering melt index. The above references are incorporated by reference.
In a conventional process for producing oriented polystyrene (OPS) virgin high molecular weight polystyrene polymer, reprocessed polymer and a small amount of high impact polystyrene (Hips) modifier resin are fed into a two stage extruder. The material is melted, mixed, degassed and extruded through a flat sheet die, typically at about 1-1.5 mm. (50-60 mils) thickness. The extruded "base" sheet is then passed through a set of cooled calendar or polishing rolls which set the surface.
The base sheet is then thermally preconditioned before being stretched 2-4 times in the Machine Direction. This is accomplished in a conventional stretch roll type Machine Direction Orientor (MDO) by passing the sheet over several sets of rolls with precise temperature control. After the sheet is preconditioned by cooling, it is stretched very rapidly between a slow speed roll and a high speed roll. The speed differential between the slow roll and the fast roll as well as the temperature of the sheet when the stretching occurs determines the Machine Direction (MD) orientation.
Once the stretching is complete the sheet is further temperature conditioned or annealed by passing it over several heat transfer rolls to control the orientation temperature conditions. This annealing is selected to preserve the predetermined level of orientation and prepares the sheet for Transverse Direction (TDO) Orientation.
The sheet then enters a conventional tenter type Transverse Direction Orientor (TDO). During this process step the sheet is mechanically grasped on its edges by clips which are attached to a continuous chain. Unlike the MDO where conduction is used to control the sheet temperature, in the TDO the sheet temperature is controlled by convection. Lateral stretching in the TD is accomplished when the distance between the two chains, which hold the sheet, is increased. During TD orientation the sheet is stretched 2-4 times as the distance between the chains is increased. After the sheet is TD oriented the edges (where the clips held the sheet) are trimmed and recycled to the extruder feed stream. The final sheet product usually has a thickness about 15-25% of the extrusion die. For thermoforming semirigid containers, for instance, a final thickness of about 0.5 mm (15-20 mils) is satisfactory.
Conventional processes for making OPS have a very narrow operating window, in that temperatures must be closely controlled (e.g., .+-.about 1.degree. C.). If the temperature is too low the sheet may stick to the MDO rolls creating a process loss and/or poor sheet visual quality. If the temperature is too high the orientation may be incorrect or thickness gauge control is sacrificed and the final product strength uniformity is unacceptable. Once the edge trim is recovered, the sheet is cooled and optionally roll-coated on both sides with a silicone water solution. This coating serves two purposes. First it provides a mold release. Once produced, the OPS sheet or web stock material can be later thermoformed into clear packaging components, stackable containers or the like. The silicone coating allows the finished parts to be separated easily from the plug assist molds. Without this coating mold jams are prevalent. Secondly, the coating acts as denesting agent. Finished OPS containers are typically stacked one on top of another, with a large amount of surface contact area. Conventional uncoated sheet produces parts may not readily separate from each other or "denest".